1. Field of the Invention
This invention relates to apparatus for distributing gas and liquid, separately and concurrently, from a common main conduit to filter underdrains for backwash of filter media.
2. Description of Related Art
In a typical gravity filter for filtering water and wastewater, one or more filter tanks are arranged in adjacent or opposing batteries. Filter underdrain laterals are laid on the floor of the filter tank one next to the other in parallel rows to define gas and liquid flow conduits below a bed of filtering media. The conduits make possible the collection of filtered liquid during filtration and the distribution of gas and liquid for backwash. A common main conduit (known as a “flume”) is located immediately next to the filter tank to collect the filtered liquid from the underdrain laterals during filtration and to distribute gas and/or liquid to the underdrain laterals during gas (typically air) or liquid (typically water) backwash. The common main conduit also distributes gas and liquid separately and/or simultaneously to each of the underdrain laterals during backwash. In some installations, the common main conduit has its floor located somewhat lower than the elevation of the filter floor or even on the same elevation as the filter floor. Consulting engineers and contractors prefer these arrangements, because they are easier and cheaper to install. In retrofitting old filters with new underdrains, it is preferable to maintain the existing flume structure to reduce costs.
U.S. Pat. Nos. 6,306,310 and 6,312,611, both to Bergmann et al. and both entitled “Apparatus for Distributing Gas and Liquid During Concurrent Gas/Liquid Backwash in Filter Underdrain Flumes” are hereby incorporated by reference in their entirety. Each of these patents is directed to providing more room in filter systems having flumes by raising the interface of gas and liquid during backwash, while at the same time providing clear passages inside the common main conduit for gas flow to the underdrain laterals.
However, in some installations, it has been found desirable to prevent or limit air from entering the backwash liquid opening into the main filter flume during concurrent gas/liquid backwash. It has also been found desirable to limit the displacement of water from the filter flume into the filter tank during concurrent air and water backwash, so that the rise of water inside the filter tank is kept under control. It has also been found desirable to further control the velocity of liquid in the primary flume to ensure uniform liquid backwash distribution.
The present invention is directed to these and other objects as will be apparent from the description below.